Systems and methods for protecting power conversion systems under open and/or short circuit conditions

ABSTRACT

System and method are provided for protecting a power converter. The system includes a first comparator, and an off-time component. The first comparator is configured to receive a sensing signal and a first threshold signal and generate a first comparison signal based on at least information associated with the sensing signal and the first threshold signal, the power converter being associated with a switching frequency and further including a switch configured to affect the primary current. The off-time component is configured to receive the first comparison signal and generate an off-time signal based on at least information associated with the first comparison signal. The off-time component is further configured to, if the first comparison signal indicates the sensing signal to be larger than the first threshold signal in magnitude, generate the off-time signal to keep the switch to be turned off for at least a predetermined period of time.

1.CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority to Chinese Patent Application No.201110362317.6, filed Nov. 8, 2011, commonly assigned, incorporated byreference herein for all purposes.

2. BACKGROUND OF THE INVENTION

The present invention is directed to integrated circuits. Moreparticularly, the invention provides systems and methods for protectinga power conversion system. Merely by way of example, the invention hasbeen applied to protecting a power conversion system with pulse-widthmodulation. But it would be recognized that the invention has a muchbroader range of applicability.

Pulse-width-modulation (PWM) technology is widely used in powerconversion systems. Various protection mechanisms, such as over-voltageprotection, over-temperature protection, current-limiting orover-current protection (OCP), and over-power protection (OPP), areoften built in circuitry associated with a PWM controller to protect apower conversion system in which the PWM controller is used frompotential damages. The protection mechanisms usually work when thecircuitry associated with the PWM controller operates in normalconditions. But when the circuitry associated with the PWM controller isunder certain conditions, the protection mechanisms often cannotfunction properly.

FIG. 1 is a simplified diagram showing a conventional power conversionsystem including a pulse-width-modulation (PWM) controller. The powerconversion system 100 includes a PWM controller 102, a power switch 124,a current-sensing resistor 126, an isolated feedback component 128, aprimary winding 130, a secondary winding 132, a capacitor 134, and arectifying diode 136. The PWM controller 102 includes a PWM component104, a logic-control component 106, a gate driver 108, a comparator 110,and a leading-edge-blanking (LEB) component 112. Further, the PWMcontroller 102 includes five terminals 114, 116, 118, 120, and 122. Forexample, the power switch 124 is a transistor.

In operation, a primary current 138 flows through the primary winding130, the power switch 124, and the current-sensing resistor 126 (e.g.,R_(s)). In response, a resistor signal 140 (e.g., V_(CS)) is output tothe terminal 118 (e.g., terminal CS). The LEB component 112 receives thesignal 140, and outputs a current-sensing signal 142 to a non-invertinginput terminal of the comparator 110. For example, the current-sensingsignal 142 is the resistor signal 140 processed by the LEB component112. In another example, the LEB component 112 can be removed, and thecurrent-sensing signal 142 is the same as the resistor signal 140.

The comparator 110 receives a threshold signal 144 (e.g., Y_(th-OC)) atan inverting input terminal, and generates a comparison signal 146 basedon the threshold signal 144 and the current-sensing signal 142. The PWMcomponent 104 receives a feedback signal 150 from the isolated feedbackcomponent 128 and the current-sensing signal 142, and in responsegenerates a modulation signal 152. The logic-control component 106receives the comparison signal 146 and the modulation signal 152, and inresponse outputs a signal 148 to the gate driver 108 for driving thepower switch 124.

Over-current protection is usually needed to limit the primary current138 in order to protect the power conversion system 100 from variousdamaging conditions, such as excessive power, thermal run-away,transformer saturation, and excessive current and voltage stress. Innormal operation, the primary current 138 is lower than a predeterminedcurrent limit (e.g., I_(Limit)) in magnitude. The predetermined currentlimit can be determined according to the following equation.

$\begin{matrix}{I_{Limit} = {{\frac{V_{in}}{L_{p}} \times t_{on}} = \frac{V_{{th} - {OC}}}{R_{s}}}} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$

where I_(Limit) represents the predetermined current limit, V_(in)represents an input voltage 154 on the primary winding 130, and L_(p)represents an inductance 156 of the primary winding 130. Additionally,t_(on) represents a time period during which the power switch 124 isclosed (e.g., on), V_(th-OC) represents the threshold signal 144, andR_(s) represents the resistance of the current-sensing resistor 126.

If the primary current 138 becomes greater than the predeterminedcurrent limit (e.g., I_(Limit)) in magnitude, the current-sensing signal142 is greater than the threshold signal 144 (e.g., V_(th-OC)) inmagnitude. In response, the PWM controller 102 turns off the powerswitch 124, and shuts down the power conversion system 100. But undercertain conditions, the power conversion system 100 often cannot beeffectively protected from being damaged or blown out.

Hence it is highly desirable to improve techniques for protecting apower conversion system.

3. BRIEF SUMMARY OF THE INVENTION

The present invention is directed to integrated circuits. Moreparticularly, the invention provides systems and methods for protectinga power conversion system. Merely by way of example, the invention hasbeen applied to protecting a power conversion system with pulse-widthmodulation. But it would be recognized that the invention has a muchbroader range of applicability.

According to one embodiment, a system for protecting a power converterincludes a first comparator and an off-time component. The firstcomparator is configured to receive a sensing signal and a firstthreshold signal and generate a first comparison signal based on atleast information associated with the sensing signal and the firstthreshold signal, the sensing signal being associated with at least aprimary current flowing through a primary winding of the powerconverter, the power converter being associated with a switchingfrequency and further including a switch configured to affect theprimary current. The off-time component is configured to receive thefirst comparison signal and generate an off-time signal based on atleast information associated with the first comparison signal. Theoff-time component is further configured to, if the first comparisonsignal indicates the sensing signal to be larger than the firstthreshold signal in magnitude, generate the off-time signal to keep theswitch to be turned off for at least a predetermined period of time, thepredetermined period of time extending beyond at least a beginning of anext switching period corresponding to the switching frequency.

According to another embodiment, a system for protecting a powerconverter includes a first comparator and a detection component. Thefirst comparator is configured to receive a first input signal and asecond input signal and generate a first comparison signal based on atleast information associated with the first input signal and the secondinput signal, the first input signal being associated with at least aprimary current flowing through a primary winding of the powerconverter, the power converter further including a switch configured toaffect the primary current. The detection component is configured toreceive the first comparison signal and generate an off-time signalbased on at least information associated with the first comparisonsignal. Further, the detection component is configured, if the firstcomparison signal indicates that the first input signal is smaller thanthe second input signal in magnitude for a first predetermined period oftime, to generate the off-time signal to turn off the switch. Moreover,the detection component is configured, if the first comparison signaldoes not indicate that the first input signal is smaller than the secondinput signal in magnitude for the first predetermined period of time,not to generate the off-time signal to turn off the switch.

According to yet another embodiment, a system for protecting a powerconverter includes a first comparator, a timing component, and anoff-time component. The first comparator is configured to receive asensing signal and a first threshold signal and generate a firstcomparison signal based on at least information associated with thesensing signal and the first threshold signal, the sensing signal beingassociated with at least a primary current flowing through a primarywinding of the power converter, the power converter being associatedwith a switching frequency and further including a switch configured toaffect the primary current. The timing component is configured toreceive an input signal and generate a timing signal. The off-timecomponent is configured to receive the timing signal, to detect thefirst comparison signal at a detection time in response to the timingsignal, and to generate an off-time signal based on at least informationassociated with the first comparison signal if the detection timecorresponds to the time when the switch is turned on. The off-timecomponent is further configured to, if the detection time corresponds tothe time when the switch is turned on and the detected first comparisonsignal indicates that the sensing signal to be smaller than the firstthreshold signal in magnitude, generate the off-time signal to keep theswitch to be turned off for at least a predetermined period of time, thepredetermined period of time extending beyond at least a beginning of anext switching period corresponding to the switching frequency. Forexample, the off-time component is further configured, if the detectiontime corresponds to a time when the switch is turned off, not togenerate the off-time signal.

According to yet another embodiment, a method for protecting a powerconverter includes: receiving a sensing signal and a first thresholdsignal, processing information associated with the sensing signal andthe first threshold signal, and generating a first comparison signalbased on at least information associated with the sensing signal and thefirst threshold signal, the sensing signal being associated with atleast a primary current flowing through a primary winding of the powerconverter, the power converter being associated with a switchingfrequency and further including a switch configured to affect theprimary current. Additionally, the method includes processinginformation associated with the first comparison signal, and outputtingan off-time signal based on at least information associated with thefirst comparison signal. Furthermore, the process for outputting anoff-time signal includes, if the first comparison signal indicates thesensing signal to be larger than the first threshold signal inmagnitude, outputting the off-time signal to keep the switch to beturned off for at least a predetermined period of time, thepredetermined period of time extending beyond at least a beginning of anext switching period corresponding to the switching frequency.

According to yet another embodiment, a method for protecting a powerconverter includes receiving a first input signal and a second inputsignal, processing information associated with the first input signaland the second input signal, and generating a first comparison signalbased on at least information associated with the first input signal andthe second input signal, the first input signal being associated with atleast a primary current flowing through a primary winding of the powerconverter, the power converter further including a switch configured toaffect the primary current. Additionally, the method includes processinginformation associated with the first comparison signal. Furthermore,the method includes if the first comparison signal indicates that thefirst input signal is smaller than the second input signal in magnitudefor a first predetermined period of time, outputting an off-time signalbased on at least information associated with the first comparisonsignal to turn off the switch, and if the first comparison signal doesnot indicate that the first input signal is smaller than the secondinput signal in magnitude for the first predetermined period of time,not outputting the off-time signal.

According to yet another embodiment, a method for protecting a powerconverter includes receiving a sensing signal and a first thresholdsignal, processing information associated with the sensing signal andthe first threshold signal, and generating a first comparison signalbased on at least information associated with the sensing signal and thefirst threshold signal, the sensing signal being related to at least aprimary current flowing through a primary winding of the powerconverter, the power converter being associated with a switchingfrequency and further including a switch configured to affect theprimary current. Additionally, the method includes receiving an inputsignal, processing information associated with the input signal, andgenerating a timing signal based on at least information associated withthe input signal. Further, the method includes receiving the timingsignal, detecting the first comparison signal at a detection time inresponse to the timing signal, and processing information associatedwith the first comparison signal. Moreover, the method includesoutputting an off-time signal based on at least information associatedwith the first comparison signal if the detection time corresponds tothe time when the switch is turned on. The process for outputting anoff-time signal includes, if the detection time corresponds to the timewhen the switch is turned on and the detected first comparison signalindicates that the sensing signal to be smaller than the first thresholdsignal in magnitude, outputting the off-time signal to keep the switchto be turned off for at least a predetermined period of time, thepredetermined period of time extending beyond at least a beginning of anext switching period corresponding to the switching frequency. Forexample, the off-time component is further configured, if the detectiontime corresponds to a time when the switch is turned off, not togenerate the off-time signal.

Depending upon embodiment, one or more benefits may be achieved. Thesebenefits and various additional objects, features and advantages of thepresent invention can be fully appreciated with reference to thedetailed description and accompanying drawings that follow.

4. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified diagram showing a conventional power conversionsystem including a pulse-width-modulation (PWM) controller.

FIG. 2 is a simplified diagram showing a power conversion systemincluding a PWM controller according to an embodiment of the presentinvention.

FIG. 3 is a simplified diagram showing a method for protecting the powerconversion system shown in FIG. 2 according to one embodiment of thepresent invention.

FIG. 4 is a simplified diagram showing the power conversion system shownin FIG. 1 that includes a chip-ground terminal in the open-circuitcondition or the floating condition.

FIG. 5 is a simplified diagram showing a power conversion systemincluding a PWM controller according to another embodiment of thepresent invention.

FIG. 6 is a simplified diagram showing the power conversion system shownin FIG. 1 that includes a current-sensing terminal in a short-circuitcondition.

FIG. 7 is a simplified diagram showing a power conversion systemincluding a PWM controller according to another embodiment of thepresent invention.

FIG. 8 is a simplified diagram showing a method for protecting the powerconversion system shown in FIG. 7 according to another embodiment of thepresent invention.

5. DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to integrated circuits. Moreparticularly, the invention provides systems and methods for protectinga power conversion system. Merely by way of example, the invention hasbeen applied to protecting a power conversion system with pulse-widthmodulation. But it would be recognized that the invention has a muchbroader range of applicability.

Referring back to FIG. 1, the magnitude of the resistor signal 140 oftendoes not reflect the magnitude of the primary current 130 under certainconditions, such as a short-circuit condition of the sensing resistor126 and an open-circuit condition or a floating condition of theterminal 122 (e.g., terminal GND). Then, the power conversion systemoften cannot be protected by the protection mechanism discussed abovefrom damages or being blown out.

In addition, in an output short-circuit condition, the power conversionsystem 100 often enters into a deep continuous conduction mode (CCM).The switching frequency of the system 100 usually reaches a maximumvalue, and the LEB time often becomes fixed. The demagnetization time isusually not long enough. The primary current 138 often reaches a highlevel, which results in transformer saturation and high voltage spikesduring the switch-off transition to damage or blow out the system 100.Additional protection mechanisms are needed under certain conditions forprotecting the power conversion system.

FIG. 2 is a simplified diagram showing a power conversion systemincluding a PWM controller according to an embodiment of the presentinvention. This diagram is merely an example, which should not undulylimit the scope of the claims. One of ordinary skill in the art wouldrecognize many variations, alternatives, and modifications.

The power conversion system 200 includes a PWM controller 202, a powerswitch 224, a current-sensing resistor 226, an isolated feedbackcomponent 228, a primary winding 230, a secondary winding 232, acapacitor 234, and a rectifying diode 236. The PWM controller 202includes a PWM component 204, a logic-control component 206, a gatedriver 208, a comparator 210, a leading edge blanking (LEB) component212, an auxiliary comparator 248, and an off-time controller 250.Further, the PWM controller 202 includes five terminals 214, 216, 218,220, and 222.

For example, the power switch 224 is a transistor. In another example,the power switch 224, the current-sensing resistor 226, the isolatedfeedback component 228, the primary winding 230, the secondary winding232, the capacitor 234, and the rectifying diode 236 are the same as thepower switch 124, the current-sensing resistor 126, the isolatedfeedback component 128, the primary winding 130, the secondary winding132, the capacitor 134, and the rectifying diode 136, respectively. Inyet another example, the PWM component 204, the logic-control component206, the gate driver 208, the comparator 210, the LEB component 212, andthe terminals 214, 216, 218, 220 and 222 are the same as the PWMcomponent 104, the logic-control component 106, the gate driver 108, thecomparator 110, the LEB component 112, and the terminals 114, 116, 118,120, and 122, respectively.

According to one embodiment, a primary current 238 flows through theprimary winding 230, the power switch 224 and the current-sensingresistor 226 (e.g., R_(s)). For example, a resistor signal 240 (e.g.,V_(CS)) is output to the LEB component 212 through the terminal 218(e.g., terminal CS). In another example, the LEB component 212generates, in response, a current-sensing signal 242. In yet anotherexample, the current-sensing signal 242 is the resistor signal 240processed by the LEB component 212. In yet another example, the LEBcomponent 212 can be removed, and the current-sensing signal 242 is thesame as the resistor signal 240. In yet another example, the comparator210 receives the current-sensing signal 242 at a non-inverting inputterminal and a threshold signal 244 (e.g., V_(th-OC)) at an invertinginput terminal, and outputs a comparison signal 254 to the logic-controlcomponent 206.

According to another embodiment, the auxiliary comparator 248 receivesthe current-sensing signal 242 at a non-inverting input terminal and anauxiliary threshold signal 245 (e.g., V_(th-OC) _(_) _(aux)) at aninverting input terminal, and generates an auxiliary comparison signal246. For example, the off-time controller 250 receives the auxiliarycomparison signal 246 and outputs an off-time signal 252 to thelogic-control component 206. In another example, the PWM component 204receives a feedback signal 256 from the isolated feedback component 228and the current-sensing signal 242, and in response outputs a modulationsignal 258 to the logic-control component 206. In yet another example,the logic-control component 206 outputs a signal 260, based on thecomparison signal 254, the off-time signal 252 and the modulation signal258, to the gate driver 208 for driving the power switch 224. In yetanother example, the auxiliary threshold signal 245 is greater than thethreshold signal 244 in magnitude. In yet another example, the auxiliarythreshold signal 245 is 20% larger than the threshold signal 244 inmagnitude. In yet another example, the auxiliary threshold signal 245 is50% larger than the threshold signal 244 in magnitude.

According to yet another embodiment, in normal operation, the primarycurrent 238 is lower than a predetermined level in magnitude. Forexample, the current-sensing signal 242 is less than the auxiliarythreshold signal 245 in magnitude. In another example, the auxiliarycomparison signal 246 is at a logic low level, and the off-timecontroller 250 does not activate a long off-time period T₁. In yetanother example, the PWM controller 202 turns on and off the powerswitch 224 normally. In yet another example, the system 200 has a normalswitching period corresponding to a normal switching frequency and anormal off-period (e.g., an off-time T₀) which is less than the longoff-time period T₁.

As shown in FIG. 2, if the output short-circuit condition occurs, theprimary current 238 often reaches a high level, and the current-sensingsignal 242 generated based on the resistor signal 240 exceeds theauxiliary threshold signal 245 in magnitude according to one embodiment.For example, the auxiliary comparison signal 246 changes from the logiclow level to a logic high level. In another example, in response, theoff-time controller 250 activates the long off-time period T₁. In yetanother example, the PWM controller 202 turns off the power switch 224for at least the long off-time period T₁. In yet another example, thelong off-time period T₁ extends beyond at least a beginning of a nextswitching period corresponding to a normal switching frequency of thesystem 200. In yet another example, the long off-time period T₁ ends inthe next normal switching period. In yet another example, the longoff-time period T₁ is no less than one normal switching period. In yetanother example, the long off-time period T₁ is determined by theoff-time controller 250.

In another embodiment, after the long off-time period T₁, the PWMcontroller 202 resumes switching on and off the power switch 224normally. For example, the PWM controller 202 turns off the power switch224 permanently and the system 200 is shut down. In another example,during the long off-time period T₁, the primary current 238 is limited.In yet another example, during the off-time period, output load receivesenergy and the primary side receives power. In yet another example, thelonger the off-time period is, the more energy the output load receives,and the more power the primary side delivers in the switching periodthat includes the off-time period. In yet another example, the longerthe off-time period is, the less the magnitude of the primary current iswhen the power switch is turned on again.

FIG. 3 is a simplified diagram showing a method for protecting the powerconversion system 200 according to one embodiment of the presentinvention. This diagram is merely an example, which should not undulylimit the scope of the claims. One of ordinary skill in the art wouldrecognize many variations, alternatives, and modifications.

The method 300 for protecting the power conversion system 200 includesat least a process 302 for detecting the peak current (e.g., the peakvalue of the current 238) cycle by cycle, a process 304 for comparingthe current-sensing signal 242 (e.g., V_(CS)) with the auxiliarythreshold signal 245 (e.g., V_(th-OC) _(_) _(aux)), a process 306 foractivating the long off-time period T₁, a process 308 for performingnormal operations with the normal off-time period (e.g., the off-timeT₀), and a process for turning on the power switch 224 again using anext PWM signal.

According to one embodiment, the current 238 is detected cycle by cycleat the process 302. For example, the current-sensing signal 242 (e.g.,V_(CS)) is compared with the auxiliary threshold signal 245 (e.g.,V_(th-OC) _(_) _(aux)) at the process 304. In another example, if thecurrent-sensing signal 242 (e.g., V_(CS)) is larger than the auxiliarythreshold signal 245 (e.g., V_(th-OC) _(_) _(aux)) in magnitude, thelong off-time period T₁is activated at the process 306. In yet anotherexample, the power switch 224 is turned off for at least the longoff-time period T₁. In yet another example, the power switch 224 isturned off permanently and the system 200 is shut down if thecurrent-sensing signal 242 is larger than the auxiliary threshold signal245 in magnitude. In yet another example, at the process 308, if thecurrent-sensing signal 242(e.g., V_(CS)) is no larger than the auxiliarythreshold signal 245 (e.g., V_(th-OC) _(_) _(aux)) in magnitude, thelong off-time period T₁ is not activated, and the system 200 operateswith the normal off-time period (e.g., the off-time T₀) which is lessthan the long off-time period T₁. In yet another example, the powerswitch 224 is turned on again by a next PWM signal at the process 310.

FIG. 4 is a simplified diagram showing the power conversion system 100that includes the terminal 122 in the open-circuit condition or thefloating condition. For example, the terminal 122 (e.g., terminal GND)is sometimes under an open-circuit condition or a floating condition. Inanother example, the resistance of the resistor 126 is often small, andthe terminal 118 (e.g., terminal CS) is often a node with a lowestimpedance compared to other terminals on the PWM controller 102. In yetanother example, a chip-supply current 162 flows into the PWM controller102 via the terminal 114 (e.g., terminal VCC). In yet another example,the current 162 flows past the terminal 122 (e.g., terminal GND) that isunder the open-circuit condition or the floating condition. In yetanother example, through an electrostatic discharge (ESD) diode 160, thecurrent 162 flows out of the PWM controller 102 via the terminal 118(e.g., terminal CS). In yet another example, the current 162 flows,through the resistor 126, to the system ground 164. In yet anotherexample, the resistor signal 140 (e.g., the voltage at the terminal 118)is lower than the chip-ground voltage 145 (e.g., the voltage at theterminal 122) by approximately a predetermined voltage V _(d) (e.g., aforward voltage drop of the ESD diode 160). Hence, the resistor signal140 does not reflect a primary current 138 that flows through theprimary winding 130 according to certain examples.

Thus, additional protection mechanisms are often needed when theterminal 122 (e.g., terminal GND) is under the open-circuit condition orthe floating condition for protecting the power conversions system fromdamaging conditions, such as over power, transformer saturation, thermalrun-away or being blown out.

FIG. 5 is a simplified diagram showing a power conversion systemincluding a PWM controller according to another embodiment of thepresent invention. This diagram is merely an example, which should notunduly limit the scope of the claims. One of ordinary skill in the artwould recognize many variations, alternatives, and modifications.

The power conversion system 500 includes a PWM controller 502, a powerswitch 524, a current-sensing resistor 526, an isolated feedbackcomponent 528, a primary winding 530, a secondary winding 532, acapacitor 534, and a rectifying diode 536. The PWM controller 502includes a PWM component 504, a logic-control component 506, a gatedriver 508, a comparator 510, a leading edge blanking (LEB) component512, an auxiliary comparator 548, a detection controller 550, and anoffset component 562. Further, the PWM controller 502 includes fiveterminals 514, 516, 518, 520, and 522.

For example, the power switch 524 is a transistor. In another example,the power switch 524, the current-sensing resistor 526, the isolatedfeedback component 528, the primary winding 530, the secondary winding532, the capacitor 534, and the rectifying diode 536 are the same as thepower switch 124, the current-sensing resistor 126, the isolatedfeedback component 128, the primary winding 130, the secondary winding132, the capacitor 134, and the rectifying diode 136, respectively. Inyet another example, the PWM component 504, the logic-control component506, the gate driver 508, the comparator 510, the LEB component 512, andthe terminals 514, 516, 518, 520 and 522 are the same as the PWMcomponent 104, the logic-control component 106, the gate driver 108, thecomparator 110, the LEB component 112, and the terminals 114, 116, 118,120 and 122, respectively.

According to one embodiment, a primary current 538 flows through theprimary winding 530, the power switch 524 and the current-sensingresistor 526 (e.g., R_(s)). For example, a resistor signal 540 (e.g.,V_(CS)) is output to the LEB component 512 through the terminal 518(e.g., terminal CS). In another example, the LEB component 512generates, in response, a current-sensing signal 542. In yet anotherexample, the current-sensing signal 542 is the resistor signal 540processed by the LEB component 512. In yet another example, the LEBcomponent 512 can be removed, and the current-sensing signal 542 is thesame as the resistor signal 540. In yet another example, the comparator510 receives the current-sensing signal 542 at a non-inverting inputterminal and a threshold signal 544 (e.g., V_(th-OC)) at an invertinginput terminal, and outputs a comparison signal 554 to the logic-controlcomponent 506.

According to another embodiment, the offset component 562 receives thecurrent-sensing signal 542 and generates a modified signal 564. Forexample, the auxiliary comparator 548 receives the modified signal 546at a non-inverting input terminal and a chip-ground voltage 545 (e.g.,the voltage of the terminal 522) at an inverting input terminal, andgenerates an auxiliary comparison signal 546. In another example, thedetection controller 550 receives the auxiliary comparison signal 546and outputs a detection signal 552 to the logic-control component 506.

According to yet another embodiment, the PWM component 504 receives afeedback signal 556 from the isolated feedback component 528 and thecurrent-sensing signal 542, and in response outputs a modulation signal558 to the logic-control component 506. For example, the logic-controlcomponent 506 generates a signal 560 based on the received comparisonsignal 554, the detection signal 552 and the modulation signal 558. Inanother example, the gate driver 508 receives the signal 560 andgenerates a drive signal 566 for driving the power switch 524.

According to yet another embodiment, if the terminal 522 (e.g., terminalGND) is under the open-circuit condition or the floating condition, theresistor signal 540 is lower than the chip-ground voltage 545 (e.g., thevoltage at the terminal 522) by approximately a predetermined voltageV_(d). For example, the predetermined voltage V_(d) is a forward voltagedrop of an ESD diode. In another example, the ESD diode is part of thePWM controller 502. In yet another example, the modified signal 564 isproportional to the sum of the current-sensing signal 542 and an offsetvoltage V₀. In yet another example, the modified signal 564 is equal tothe current-sensing signal 542 plus the offset voltage V₀. In yetanother example, the offset voltage V₀ is larger than zero and less thanthe predetermined voltage V_(d). In yet another example, the modifiedsignal 564 is lower than the chip-ground voltage 545 if the terminal 522(e.g., terminal GND) is under the open-circuit condition or the floatingcondition. In yet another example, the auxiliary comparison signal 546is at a logic high level if the modified signal 564 is less than thechip-ground voltage 545.

According to yet another embodiment, if the terminal 522 (e.g., terminalGND) is not under the open-circuit condition or the floating condition,the modified signal 564 that is equal to the current-sensing signal 542(e.g., V_(CS)) plus an offset voltage V₀ is greater than the chip-groundvoltage 545 (e.g., the voltage of the terminal 522). For example, theauxiliary comparison signal 546 is at a logic low level if the modifiedsignal 564 is greater than the chip-ground voltage 545.

As shown in FIG. 5, if the detection controller 550 detects theauxiliary comparison signal 546 to be at the logic low level whichindicates that the terminal 522 is not under the open-circuit conditionor the floating condition, the detection controller 550 outputs thedetection signal 552 in order to keep the PWM controller 502 operatingnormally according to one embodiment. For example, if the detectioncontroller 550 detects the auxiliary comparison signal 546 changes fromthe logic low level to the logic high level, the detection controller550 continues to monitor the auxiliary comparison signal 546 for apredetermined time period T₂. In another example, if during thepredetermined time period T₂, the auxiliary comparison signal 546maintains at the logic high level, the detection controller 550 outputsthe detection signal 552 in order to turn off the power switch 524 forat least a time period T₃. In yet another example, after the time periodT₃, the PWM controller 502 resumes turning on and off the power switch524 normally. In yet another example, in response to the signal 560, thePWM controller 502 turns off the power switch 524 permanently and thesystem 500 is shut down. In yet another example, the time period T₃extends beyond at least a beginning of a next switching periodcorresponding to a switching frequency of the system 500. In yet anotherexample, the time period T₃ ends in the next switching period. In yetanother example, the time period T₃ is no less than one switchingperiod.

In another embodiment, if during the predetermined time period T₂, theauxiliary comparison signal 546 changes from the logic high level to thelogic low level, the detection controller 550 does not change thedetection signal 552 until the auxiliary comparison signal 546 changesto the logic high level again and remains at the logic high level for atime period equal to T₃. For example, the predetermined time period T₂is in the range of micro seconds.

FIG. 6 is a simplified diagram showing the power conversion system 100that includes the terminal 118 in a short-circuit condition. Forexample, the terminal 118 (e.g., terminal CS) is sometimes under ashort-circuit condition (e.g., the resistor 126 is shorted). In anotherexample, a resistor signal 140 (e.g., V_(CS)) is approximately equal tothe voltage of the system ground 164 (e.g., 0 V). Hence, the resistorsignal 140 (e.g., V_(CS)) does not reflect a primary current 138 thatflows through the primary winding 130 according to certain examples.

Additional protection mechanisms are often needed when the terminal 118(e.g., terminal CS) is under the short-circuit condition (e.g., theresistor 126 is shorted) for protecting the power conversion system fromdamaging conditions, such as over power, transformer saturation, thermalrun-away or being blown out.

FIG. 7 is a simplified diagram showing a power conversion systemincluding a PWM controller according to another embodiment of thepresent invention. This diagram is merely an example, which should notunduly limit the scope of the claims. One of ordinary skill in the artwould recognize many variations, alternatives, and modifications.

The power conversion system 700 includes a PWM controller 702, a powerswitch 724, a current-sensing resistor 726, an isolated feedbackcomponent 728, a primary winding 730, a secondary winding 732, acapacitor 734, and a rectifying diode 736. The PWM controller 702includes a PWM component 704, a logic-control component 706, a gatedriver 708, a comparator 710, a leading edge blanking (LEB) component712, an auxiliary comparator 748, a detection component 750, a timer768, and an off-time controller 770. Further, the PWM controller 702includes five terminals 714, 716, 718, 720, and 722.

For example, the power switch 724 is a transistor. In another example,the power switch 724, the current-sensing resistor 726, the isolatedfeedback component 728, the primary winding 730, the secondary winding732, the capacitor 734, and the rectifying diode 736 are the same as thepower switch 124, the current-sensing resistor 126, the isolatedfeedback component 128, the primary winding 130, the secondary winding132, the capacitor 134, and the rectifying diode 136, respectively. Inyet another example, the PWM component 704, the logic-control component706, the gate driver 708, the comparator 710, the LEB component 712, andthe terminals 714, 716, 718, 720 and 722 are the same as the PWMcomponent 104, the logic-control component 106, the gate driver 108, thecomparator 110, the LEB component 112, and the terminals 114, 116, 118,120 and 122, respectively.

According to one embodiment, a primary current 738 flows through theprimary winding 730, the power switch 724 and the current-sensingresistor 726 (e.g., R_(s)). For example, a resistor signal 740 (e.g.,V_(CS)) is output to the LEB component 712 through the terminal 718(e.g., terminal CS). In another example, the LEB component 712 generatesa current-sensing signal 742. In yet another example, thecurrent-sensing signal 742 is the resistor signal 740 processed by theLEB component 712. In yet another example, the LEB component 712 can beremoved, and the current-sensing signal 742 is the same as the resistorsignal 740. In yet another example, the comparator 710 receives thecurrent-sensing signal 742 at a non-inverting input terminal and athreshold signal 744 (e.g., V_(th-OC)) at an inverting input terminal,and outputs a comparison signal 754 to the logic-control component 706.

According to another embodiment, the auxiliary comparator 748 receivesthe current-sensing signal 742 at a non-inverting input terminal and anauxiliary threshold signal 762 (e.g., UVP) at an inverting inputterminal, and outputs an auxiliary comparison signal 746 to thedetection component 750. For example, the timer 768 receives a clocksignal 772, and outputs a delay signal 774 to the detection component750. In another example, in response to the delay signal 774, thedetection component 750 detects the auxiliary comparison signal 746after a predetermined delay from the time when the power switch 724 isturned on.

According to yet another embodiment, the detection component 750 detectsthe auxiliary comparison signal 746 in a switching period after thepower switch 724 is turned on. For example, the switching periodincludes an on-time period and an off-time period for the power switch724. In another example, the signal 774 has the same rising edge as theclock signal 772 (e.g., the rising edge of the clock signal 772representing the beginning of the on-time period for the power switch724). In yet another example, the detection component 750 detects theauxiliary comparison signal 746 at a falling edge of the signal 774. Inyet another example, the falling edge of the signal 774 followsimmediately, after the predetermined delay, a rising edge of the signal774.

As another example, if the detection component 750 detects the auxiliarycomparison signal 746 during the off-time period for the power switch724, the output of the detection component 750 is not active. On theother hand, if the detection component 750 detects the auxiliarycomparison signal 746 during the on-time period for the power switch724, the detection component 750 outputs a detection signal 776 to theoff-time controller 770, according to certain embodiments.

In another embodiment, the off-time controller 770 outputs an off-timesignal 752 to the logic-control component 706 in response to thedetection signal 776. For example, the PWM component 704 receives afeedback signal 756 from the isolated feedback component 728 and thecurrent-sensing signal 742, and in response outputs a modulation signal758 to the logic-control component 706. In another example, thelogic-control component 706 generates a signal 760 based on thecomparison signal 754, the off-time signal 752 and the modulation signal758. In yet another example, the gate driver 708 receives the signal 760and generates a drive signal 766 for driving the power switch 724.

In one embodiment, if the terminal 718 (e.g., terminal CS) is under theshort-circuit condition (e.g., the resistor 726 is shorted), theresistor signal 740 is approximately equal to the voltage of the systemground 764 (e.g., 0 V) in magnitude. For example, if the terminal 718(e.g., terminal CS) is under a nearly short-circuit condition (e.g., theterminal 718 has a very small impedance), the resistor signal 740 has asmall magnitude. In another example, the current-sensing signal 742 isless than the auxiliary threshold signal 762 in magnitude. In yetanother example, the auxiliary comparison signal 746 is at a logic lowlevel if the current-sensing signal 742 is lower than the auxiliarythreshold signal 762 in magnitude.

In another embodiment, if the terminal 718 (e.g., terminal CS) is notunder the short-circuit condition, the resistor signal 740 is greaterthan the voltage of the system ground 764 (e.g., 0 V) in magnitude afterthe predetermined delay from the time when the power switch 724 isturned on. For example, the current-sensing signal 742 (e.g., V_(CS)) isgreater than the auxiliary threshold signal 762 (e.g., UVP) in magnitudeafter the predetermined delay from the time when the power switch 724 isturned on. In another example, the auxiliary comparison signal 746 is ata logic high level if the current-sensing signal 742 is greater than theauxiliary threshold signal 762 in magnitude.

As shown in FIG. 7, if the detection component 750 detects, after thepredetermined delay from the time when the power switch 724 is turned on(e.g., at the falling edge of the signal 774), that the auxiliarycomparison signal 746 is at the logic high level, the PWM controller 702operates normally according to one embodiment. For example, the off-timecontroller 770 does not activate a long off-time period T₅. In anotherexample, the system 700 has a normal switching period corresponding to anormal switching frequency of the system 200 and a normal off-timeperiod (e.g., an off-time T₄) which is less than the long off-timeperiod T₅.

According to another embodiment, if the detection component 750 detects,after the predetermined delay (e.g., at the falling edge of the signal774), that the auxiliary comparison signal 746 is at the logic lowlevel, the detection component 750 outputs the detection signal 776 toactivate the long off-time period T₅. For example, the long off-timeperiod T₅ is determined by the off-time controller 770. In anotherexample, the long off-time period T₅ is in the range of tens of microseconds to several milliseconds. In yet another example, the PWMcontroller 702 turns off the power switch 724 for at least the longoff-time period T₅. In yet another example, the long off-time period T₅extends beyond at least a beginning of a next switching periodcorresponding to a normal switching frequency of the system 700. In yetanother example, the long off-time period T₅ ends in the next normalswitching period. In yet another example, the long off-time period T₅ isno less than one normal switching period.

In yet another example, after the long off-time period T₅, the PWMcontroller 702 resumes switching on and off the power switch 724normally. In yet another example, the PWM controller 702 turns off thepower switch 724 permanently and the system 700 is shut down. In yetanother example, during the long off-time period T₅, the primary current738 is limited. In yet another example, during the long off-time periodT₅, the switching frequency of the system 700 is reduced. In yet anotherexample, during the long off-time period T₅, power delivered to outputload is limited.

FIG. 8 is a simplified diagram showing a method for protecting the powerconversion system 700 according to one embodiment of the presentinvention. This diagram is merely an example, which should not undulylimit the scope of the claims. One of ordinary skill in the art wouldrecognize many variations, alternatives, and modifications.

The method 800 includes at least a process 802 for detecting the peakcurrent (e.g., the peak value of the current 738) cycle by cycle, aprocess 804 for comparing the current-sensing signal 742 (e.g., V_(CS))with the auxiliary threshold signal 762 (e.g., UVP) after apredetermined delay from the time when the power switch 724 is turned on(e.g., at a falling edge of the signal 774), a process 806 foractivating the long off-time period T₅, a process 808 for performingnormal operations with a normal off-time period (e.g., the off-time T₄),and a process for turning on the power switch 724 again using a next PWMsignal.

According to one embodiment, the current 738 is detected cycle by cycleat the process 802. For example, the current-sensing signal 742 (e.g.,V_(CS)) is compared with the auxiliary threshold signal 762 (e.g., UVP)after the delay (e.g., at the falling edge of the signal 774) at theprocess 804. In another example, if the current-sensing signal 742(e.g., V_(CS)) is less than the auxiliary threshold signal 762 (e.g.,UVP) in magnitude after the predetermined delay from the time when thepower switch 724 is turned on, the long off-time period T₅ is activatedat the process 806. In yet another example, the power switch 724 isturned off for at least the long off-time period T₅. In yet anotherexample, the power switch 724 is turned off permanently and the system700 is shut down if the current-sensing signal 742 is less than theauxiliary threshold signal 762 in magnitude after the predetermineddelay from the time when the power switch 724 is turned on. In yetanother example, at the process 808, if the current-sensing signal 742(e.g., V_(CS)) is not less than the auxiliary threshold signal 762(e.g., UVP) in magnitude after the predetermined delay from the timewhen the power switch 724 is turned on, the long off-time period T₅ isnot activated, and the system 700 operates with the normal off-timeperiod (e.g., the off-time T₄) which is less than the long off-timeperiod T₅. In yet another example, the power switch 724 is turned onagain by a next PWM signal at the process 810.

According to another embodiment, a system for protecting a powerconverter includes a first comparator and an off-time component. Thefirst comparator is configured to receive a sensing signal and a firstthreshold signal and generate a first comparison signal based on atleast information associated with the sensing signal and the firstthreshold signal, the sensing signal being associated with at least aprimary current flowing through a primary winding of the powerconverter, the power converter being associated with a switchingfrequency and further including a switch configured to affect theprimary current. The off-time component is configured to receive thefirst comparison signal and generate an off-time signal based on atleast information associated with the first comparison signal. Theoff-time component is further configured to, if the first comparisonsignal indicates the sensing signal to be larger than the firstthreshold signal in magnitude, generate the off-time signal to keep theswitch to be turned off for at least a predetermined period of time, thepredetermined period of time extending beyond at least a beginning of anext switching period corresponding to the switching frequency. Forexample, the system is implemented according to at least FIG. 2 and/orFIG. 3.

According to yet another embodiment, a system for protecting a powerconverter includes a first comparator and a detection component. Thefirst comparator is configured to receive a first input signal and asecond input signal and generate a first comparison signal based on atleast information associated with the first input signal and the secondinput signal, the first input signal being associated with at least aprimary current flowing through a primary winding of the powerconverter, the power converter further including a switch configured toaffect the primary current. The detection component is configured toreceive the first comparison signal and generate an off-time signalbased on at least information associated with the first comparisonsignal. Further, the detection component is configured, if the firstcomparison signal indicates that the first input signal is smaller thanthe second input signal in magnitude for a first predetermined period oftime, to generate the off-time signal to turn off the switch. Moreover,the detection component is configured, if the first comparison signaldoes not indicate that the first input signal is smaller than the secondinput signal in magnitude for the first predetermined period of time,not to generate the off-time signal to turn off the switch. For example,the system is implemented according to at least FIG. 5.

According to yet another embodiment, a system for protecting a powerconverter includes a first comparator, a timing component, and anoff-time component. The first comparator is configured to receive asensing signal and a first threshold signal and generate a firstcomparison signal based on at least information associated with thesensing signal and the first threshold signal, the sensing signal beingassociated with at least a primary current flowing through a primarywinding of the power converter, the power converter being associatedwith a switching frequency and further including a switch configured toaffect the primary current. The timing component is configured toreceive an input signal and generate a timing signal. The off-timecomponent is configured to receive the timing signal, to detect thefirst comparison signal at a detection time in response to the timingsignal, and to generate an off-time signal based on at least informationassociated with the first comparison signal if the detection timecorresponds to the time when the switch is turned on. The off-timecomponent is further configured to, if the detection time corresponds tothe time when the switch is turned on and the detected first comparisonsignal indicates that the sensing signal to be smaller than the firstthreshold signal in magnitude, generate the off-time signal to keep theswitch to be turned off for at least a predetermined period of time, thepredetermined period of time extending beyond at least a beginning of anext switching period corresponding to the switching frequency. As anexample, the off-time component is further configured, if the detectiontime corresponds to a time when the switch is turned off, not togenerate the off-time signal. For example, the system is implementedaccording to at least FIG. 7 and/or FIG. 8.

In another embodiment, a method for protecting a power converterincludes: receiving a sensing signal and a first threshold signal,processing information associated with the sensing signal and the firstthreshold signal, and generating a first comparison signal based on atleast information associated with the sensing signal and the firstthreshold signal, the sensing signal being associated with at least aprimary current flowing through a primary winding of the powerconverter, the power converter being associated with a switchingfrequency and further including a switch configured to affect theprimary current. Additionally, the method includes processinginformation associated with the first comparison signal, and outputtingan off-time signal based on at least information associated with thefirst comparison signal. Furthermore, the process for outputting anoff-time signal includes, if the first comparison signal indicates thesensing signal to be larger than the first threshold signal inmagnitude, outputting the off-time signal to keep the switch to beturned off for at least a predetermined period of time, thepredetermined period of time extending beyond at least a beginning of anext switching period corresponding to the switching frequency. Forexample, the method is implemented according to at least FIG. 2 and/orFIG. 3.

In yet another embodiment, a method for protecting a power converterincludes receiving a first input signal and a second input signal,processing information associated with the first input signal and thesecond input signal, and generating a first comparison signal based onat least information associated with the first input signal and thesecond input signal, the first input signal being associated with atleast a primary current flowing through a primary winding of the powerconverter, the power converter further including a switch configured toaffect the primary current. Additionally, the method includes processinginformation associated with the first comparison signal. Furthermore,the method includes if the first comparison signal indicates that thefirst input signal is smaller than the second input signal in magnitudefor a first predetermined period of time, outputting an off-time signalbased on at least information associated with the first comparisonsignal to turn off the switch, and if the first comparison signal doesnot indicate that the first input signal is smaller than the secondinput signal in magnitude for the first predetermined period of time,not outputting the off-time signal. For example, the method isimplemented according to at least FIG. 5.

In yet another embodiment, a method for protecting a power converterincludes receiving a sensing signal and a first threshold signal,processing information associated with the sensing signal and the firstthreshold signal, and generating a first comparison signal based on atleast information associated with the sensing signal and the firstthreshold signal, the sensing signal being related to at least a primarycurrent flowing through a primary winding of the power converter, thepower converter being associated with a switching frequency and furtherincluding a switch configured to affect the primary current.Additionally, the method includes receiving an input signal, processinginformation associated with the input signal, and generating a timingsignal based on at least information associated with the input signal.Further, the method includes receiving the timing signal, detecting thefirst comparison signal at a detection time in response to the timingsignal, and processing information associated with the first comparisonsignal. Moreover, the method includes outputting an off-time signalbased on at least information associated with the first comparisonsignal if the detection time corresponds to the time when the switch isturned on. The process for outputting an off-time signal includes, ifthe detection time corresponds to the time when the switch is turned onand the detected first comparison signal indicates that the sensingsignal to be smaller than the first threshold signal in magnitude,outputting the off-time signal to keep the switch to be turned off forat least a predetermined period of time, the predetermined period oftime extending beyond at least a beginning of a next switching periodcorresponding to the switching frequency. As an example, the off-timecomponent is further configured, if the detection time corresponds to atime when the switch is turned off, not to generate the off-time signal.For example, the method is implemented according to at least FIG. 7and/or FIG. 8.

For example, some or all components of various embodiments of thepresent invention each are, individually and/or in combination with atleast another component, implemented using one or more softwarecomponents, one or more hardware components, and/or one or morecombinations of software and hardware components. In another example,some or all components of various embodiments of the present inventioneach are, individually and/or in combination with at least anothercomponent, implemented in one or more circuits, such as one or moreanalog circuits and/or one or more digital circuits. In yet anotherexample, various embodiments and/or examples of the present inventioncan be combined. In yet another example, the auxiliary comparator 248,the off-time controller 250, the auxiliary comparator 548, the offsetcomponent 562, the detection controller 550, the auxiliary comparator748, the detection component 750, the timer 768, and the off-timecontroller 770 can be combined into a PWM controller for protecting apower conversion system.

Although specific embodiments of the present invention have beendescribed, it will be understood by those of skill in the art that thereare other embodiments that are equivalent to the described embodiments.Accordingly, it is to be understood that the invention is not to belimited by the specific illustrated embodiments, but only by the scopeof the appended claims.

1.-5. (canceled)
 6. A system for protecting a power converter, thesystem comprising: a first comparator configured to receive a firstinput signal and a second input signal and generate a first comparisonsignal based on at least information associated with the first inputsignal and the second input signal, the first input signal beingassociated with at least a primary current flowing through a primarywinding of the power converter, the power converter further including aswitch configured to affect the primary current; and a detectioncomponent configured to receive the first comparison signal and generatean off-time signal based on at least information associated with thefirst comparison signal; wherein the detection component configured: ifthe first comparison signal indicates that the first input signal issmaller than the second input signal in magnitude for a firstpredetermined period of time, to generate the off-time signal to turnoff the switch; if the first comparison signal does not indicate thatthe first input signal is smaller than the second input signal inmagnitude for the first predetermined period of time, not to generatethe off-time signal to turn off the switch.
 7. The system of claim 6wherein the first input signal is proportional to a sum of a sensingsignal and an offset voltage.
 8. The system of claim 7 wherein theoffset voltage is larger than zero in magnitude, and less than a forwardvoltage drop of an electrostatic discharge diode in magnitude.
 9. Thesystem of claim 7, and further comprising: a second comparatorconfigured to receive the sensing signal and a threshold signal andgenerate a second comparison signal based on at least informationassociated with the sensing signal and the threshold signal; wherein thesecond comparator is further configured to, if the sensing signal islarger than the threshold signal in magnitude, generate the secondcomparison signal to turn off the switch.
 10. The system of claim 6wherein: the first comparator and the detection component are on a chip;the chip includes a ground terminal; and the second input signal isequal to a voltage of the ground terminal in magnitude.
 11. The systemof claim 6 wherein the detection component is further configured, if thefirst comparison signal indicates that the first input signal is largerthan the second input signal in magnitude, not to generate the off-timesignal to turn off the switch.
 12. The system of claim 6 wherein thedetection component is further configured, if the first comparisonsignal indicates that the first input signal is smaller than the secondinput signal in magnitude for the first predetermined period of time, togenerate the off-time signal to keep the switch to be turned off for atleast a second predetermined period of time.
 13. The system of claim 12wherein the second predetermined period of time ends in a currentswitching period corresponding to the switching frequency of the powerconverter.
 14. The system of claim 12 wherein the second predeterminedperiod of time extends beyond at least a beginning of a next switchingperiod corresponding to the switching frequency of the power converter.15.-24. (canceled)
 25. A method for protecting a power converter, themethod comprising: receiving a first input signal and a second inputsignal; processing information associated with the first input signaland the second input signal; generating a first comparison signal basedon at least information associated with the first input signal and thesecond input signal, the first input signal being associated with atleast a primary current flowing through a primary winding of the powerconverter, the power converter further including a switch configured toaffect the primary current; processing information associated with thefirst comparison signal; and if the first comparison signal indicatesthat the first input signal is smaller than the second input signal inmagnitude for a first predetermined period of time, outputting anoff-time signal based on at least information associated with the firstcomparison signal to turn off the switch; and if the first comparisonsignal does not indicate that the first input signal is smaller than thesecond input signal in magnitude for the first predetermined period oftime, not outputting the off-time signal.
 26. (canceled)